In the manufacture of transparent containers of glass composition, for example, it is heretofore been proposed to measure sidewall thickness of the containers in order to detect thin areas that may affect ability of the containers to withstand pressurization and handling. In one container sidewall thickness gauge commercially employed by applicants' assignee, a radio frequency electrode is placed near the outer surface of the container wall, and the amplitude of the signal received at a coaxial pick-up electrode is related to container wall thickness. This technique is highly sensitive to distance between the pick-up electrode and the container wall surface, and the electrode is mounted on a wheel that rides on the container surface in order to control the electrode/surface separation. Mechanical contact with the container, and vibration at the probe when the container moves into position, cause a high failure rate at the electrode assembly.
European published application No. 0320139A2 discloses an electro-optical non-contact system for measuring sidewall thickness of transparent containers A laser beam is directed to intersect the container outer wall surface at an angle of 37.5.degree. to the container radius. A portion of the light beam is reflected from the outer surface of the container, and a portion is refracted into the container wall and is incident on the inner surface of the container wall. A portion of the light incident on the inner wall surface is again reflected to the outer wall surface and refracted out of the container sidewall. A fresnel lens is disposed to direct light reflected from the inner and outer sidewall surfaces onto a linear array optical sensor, at which separation between the rays reflected from the inner and outer sidewall surface reflection points is proportional to sidewall thickness. The lens and sensor are positioned so that the outer surface reflection point and the virtual image of the inner surface reflection point are in the object plane of the lens, while the sensor array is in the image plane of the lens. That is to say that the linear array sensor is optically parallel to a line between the outer surface reflection point and the virtual image of the inner surface reflection point.
Although the technique disclosed in the noted European application reduces measurement sensitivity to deviations of the inner wall surface from parallelism with the outer wall surface, it is highly sensitive to container position. That is, the disclosed technique is highly sensitive to separation between the measurement optics and the container wall surface, which is difficult to control in high-speed conveyor and inspection systems for mass production of transparent containers. It is therefore a general object of the present invention to provide an electro-optical apparatus and method for non-contact measurement of sidewall thickness in transparent containers that are substantially independent of container position relative to the measurement optics. Another object of the present invention is to provide non-contact sidewall thickness measurement apparatus and method that achieve the foregoing objects while maintaining reduced sensitivity to non-parallelism between the inner and outer sidewall surfaces. A further object of the present invention is to provide a transparent container sidewall thickness measurement apparatus and method of the described character that provide an output that simulates the output of the current radio frequency measurement gauge described above, so that the measurement apparatus may be employed in current container inspection systems without substantial modification to the remainder of the system.